Understanding the Risks of Ebola, and What ‘Direct Contact’ Means

Can you catch Ebola on a crowded bus or train if you are standing next to someone who is infected? What if that person sneezes or coughs on you? If the person has symptoms, the answer could be yes.

Questions that may have seemed theoretical a few days ago have taken on a troubling reality since a traveler from Liberia, Thomas E. Duncan, carried the virus to Texas and potentially exposed dozens of people before he was placed in isolation. And the hospital where he first sought help failed to make the diagnosis, leaving him in the community for several days when he was becoming more and more contagious.

It could happen again, said Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention, in a telephone news briefing on Thursday. He said there was no way to detect the disease during the incubation period — the interval before symptoms set in — so other infected people could pass fever checks at airports in West Africa, just as Mr. Duncan did, and board planes to the United States.

But Dr. Frieden emphasized that the odds of contracting Ebola in the United States were still extremely low. The disease is not contagious during the incubation period, and patients do not transmit it until they develop symptoms, he said. And those with symptoms will probably feel sick enough to stay home. People are highly unlikely to catch the disease on the bus or subway, Dr. Frieden said.

The incubation period can run from two to 21 days, but is usually eight to 10. The first symptoms are typically fever, aches and pains, nausea, vomiting and diarrhea. Even after they set in, it takes direct contact with bodily fluids to transmit Ebola. Blood, vomit, urine and diarrhea from very sick patients are highly infectious, but other body fluids like sweat, saliva, tears, semen and breast milk are also risky.

Direct contact means that the fluids splash or spray into someone else’s mouth, eyes or nose, or enter the bloodstream through cuts or breaks in the skin.

People can also contract the disease by touching infected fluids and then touching their eyes or mouth. The virus does not spread through the air, as measles and chickenpox do. And Ebola does not invade healthy skin, so merely touching secretions does not mean an infection will follow. But washing hands or using an alcohol-based hand sanitizer is essential after any potential contact, to avoid spreading the virus to other people or to one’s own eyes or mouth.

Ebola does not cause respiratory problems, but a cough from a sick patient could infect someone close enough to be sprayed with droplets of mucus or saliva. Droplets can generally fly about three feet, so people dealing with anyone who may be ill are told to stand at least three feet away, preferably six. Being within three feet of a patient for a prolonged time, without wearing protective gear, is considered direct contact, Dr. Frieden said.

According to the C.D.C., the virus can survive for a few hours on dry surfaces like doorknobs and countertops. But it can survive for several days in puddles or other collections of body fluid at room temperature. It is not clear how long it may survive in soiled linens and clothing. Bleach solutions can kill it.

Dr. Frieden and other experts have been saying for months that even if Ebola did reach the United States, it would not gain a foothold, because hospitals here would quickly diagnose it and would follow rigorous infection-control procedures that would prevent the disease from spreading to health workers or other patients.

An American woman who traveled in Uganda contracted the Marburg virus, similar to Ebola, and was treated in an American hospital in 2008 without spread of the disease.

Experts also say the disease would be quickly stamped out here because health authorities would trace all of a patient’s contacts and isolate any who developed symptoms.

But Mr. Duncan’s case reveals a gap in the defenses: Texas Health Presbyterian Hospital failed to diagnose the disease when he first went there last Thursday night, even though his symptoms were suggestive of Ebola and he said he had recently arrived from Liberia. He returned to the apartment where he was staying, potentially infecting those around him as well as the ambulance crew that wound up rushing him back to the same hospital on Sunday.

Although there is no proven antiviral drug to treat Ebola, supportive care to nurse people through the worst of its symptoms — tremendous loss of fluids and electrolytes like potassium — can greatly improve the odds of survival. So doctors believe the death rate here would be far lower than the devastating 70 percent in West Africa.

Whether he knew it or not, boarding that plane in Monrovia was Mr. Duncan’s best hope of saving his own life.

A version of this article appears in print on , on Page A16 of the New York edition with the headline: Understanding the Risks of Ebola, and What ‘Direct Contact’ Means. Order Reprints | Today’s Paper | Subscribe